Pressure transducers based on strain gauges fixed to an active mechanical element that undergoes a deformation (dilation, deflection, etc.) under the influence of pressure are well known. This type of transducer is particularly well suited for measurements of elevated or high pressures inside pipelines, storage vessels or processing chambers. However, up to now, they have been all based on bonded resistance strain gauges, characterized by the gauge factor G.sub.F : ##EQU1## where: R.sub.0 is electrical resistance of a strain gauge; and
.epsilon. is longitudinal strain .DELTA.l/l.sub.0. PA1 a cylindrical hollow body of predetermined length and acting as a pressure transducer, the hollow body having a central axis, cylindrical inner and outer surfaces, an end, and an opposite end provided with an inlet by which the fluid can get inside the hollow body, the ends being free to move longitudinally with respect to the central axis such that longitudinal and circumferential strains are generated in the hollow body when the pressure is applied on the inner surface, thereby producing a dilation of the hollow body; and PA1 a highly birefringent optical fiber which can maintain only some polarization states, the birefringent optical fiber having a sensing portion bonded with bonding means to the outer surface of the hollow body along a longitudinal path such that birefringence of the sensing portion changes when the sensing portion is subjected to the dilation of the hollow body, the sensing portion having a length smaller than the length of the hollow body, which is selected in function of a desired sensitivity; PA1 transmitting a polarized light beam in a form of two polarization eigenstates each parallel to one of two parallel principal birefringence axes of said birefringent optical fiber, the state of polarization of the input light beam being strain-modulated after a passage in the portion of the highly birefringent optical fiber due to change in birefringence of the birefringent optical fiber as a result of the dilation; PA1 collecting the strain-modulated output light beam; PA1 measuring change in the state of polarization between the input light beam and the strain-modulated output light beam; and PA1 determining a value indicative of the pressure as a function of the measured change in the state of polarization.
The actual deformation of the element and the strain resulting from it at the location of the strain gauge will obviously depend on the design of the active element. For higher pressures up to 100 MPa this element most often takes the form of a dilating cylinder. The cylinder is usually closed at one end and has its other end directly connected to a pressure apparatus in an arrangement that may generate some unwanted stress in the active area of the cylinder.
To suppress this unwanted stress, a pressure transducer based on a dilating cylinder with free ends has been proposed by one of the inventors, Mr. Roland Wisnienski in a publication entitled "POMIARY, AUTOMATIKA, KONTROLA" (1986) No. 3, page 60, to be used with an electrical strain gauge for measuring pressure up to 100 MPa. The deformation of such a dilating cylinder is totally independent of the stress induced by connecting it to the pressure apparatus and so depends exclusively on the value of the internal pressure delivered from the external pressure apparatus.
However, electrical strain gauges, although widely used, suffer from significant temperature drift (thermally induced voltages caused by thermocoupling and temperature effects on gauge resistance and gauge factor). In addition, the low electrical output level of such a strain gauge makes it extremely susceptible to electromagnetic interference (EMI), especially in noisy industrial environments. Desensitization of these gauges to EMI is very difficult and not always possible, and the procedure is actually more costly than resistance-strain sensing technology itself.
An object of the invention is to replace the standard electrical resistance strain gauge in the strain-gauge pressure manometer based on a dilating cylindrical element by an optical fiber.
Another object of the invention is to provide a new fiber-optic strain gauge manometer and a method thereof for measuring pressure of a fluid inside a dilating cylindrical element up to 100 MPa.
Still another object of the invention is to provide a fiber-optic strain gauge manometer which is inherently immune to electromagnetic interferences, safe in electrically dangerous or explosive environments and have a significantly greater sensitivity over prior art manometers.
Still another object of the invention is to provide a method for measuring a pressure which is directly compatible with optical data transmission systems and optical multiplexing technology.